PDF services have stopped working for me. Please help me figure out how to fix them.

Background

Clicking on the PDF menu in a print dialog box brings up a menu with three sections.

[list=] The top section has three options: Open PDF in Preview, Save as PDF…, and Save as Postscript… The middle section displays available PDF workflows. These are located in either ~/Library/PDF Services or an equivalent folder in the top-level library. Some of these PDF workflows get added automatically by applications; others are added manually by the user. Choosing any of these workflows creates a PDF and either saves it in a specified folder or does something further with it, such as attaching it to a new email message. The bottom section provides an Edit Menu… option. Selecting this option presents a drop-down list of Printing Workflows,* along with + and – buttons. I think the most common use of this is to click the + button and then select a folder in the resulting dialog box. That places an alias to the selected folder in ~/Library/PDF Services. Then, when you select that folder in the middle section of the PDF menu (see above), a PDF is created and saved to that folder.[/list]

*I'm not sure what the difference is between PDF workflows and Printing Workflows.

Problem

Only the top section of the PDF menu is working correctly. When I select a PDF workflow in the middle section of the menu, the document simply prints and no PDF is corrected. The bottom section allows me to add or subtract printing workflows but any printing workflows I add do not work. That is, they show up in the PDF workflows section of the PDF menu but they don't do anything; selecting any of them simply cancels the PDF functionality and prints the document.